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Re: What purpose do those membranes serve?

I'm not a rocket scientist or anything, but from the distant memories I have of physics class, you need a tight seal to form a vacuum, which is how the suction happens. The membrane forms the seal and then when the pump stops sucking the air out of the sealed "system," the membrane flaps open easily to let the milk into the bottle. That's why the pump doesn't work so well if they're damaged. Not a terribly scientific explanation, but that's how I understand it.